1. Field of the Invention
The present invention relates to an assembling apparatus and an assembling method of a nuclear fuel assembly, and more specifically to a guiding apparatus which enables the insertion of fuel rods to be carried out efficiently and free of surface scratches.
2. Technical Background of the Invention
In general, pressurized light water nuclear reactors utilize a type of fuel assembly in which a number of fuel rods are arranged horizontally at regular intervals. The fuel assembly comprises an upper nozzle, a lower nozzle and a plurality of fuel rod supporting grids (shortened to grids henceforth), made of thin metal straps intersecting orthognally to form grid cells. Fuel rod-guide pipes and instrumentation pipes are inserted into the grid cells, and are fixed to the grids and also to the upper and the lower nozzles at the ends. Fuel rods are also inserted into and are supported elastically in the grid cells by means of springs formed inside the grid cells.
When assembling such a fuel assembly, there is a danger of introducing scratches on the surface of the fuel rods if the rods are inserted into the grid cells as they are, because of the presence of the fuel rod supporting springs and dimples which are formed inside the grid cells. To prevent such a surface damage to the fuel rods, the springs must temporarily be deactivated while the fuel rods are being inserted into the grid cells. However, the technology to successively deactivate a large number of springs formed on each strap in the assembly required a high precision in the tooling and advanced drive-control technology.
In view of such problems in the existing technology, there have been, prior to the present invention, alternative proposed techniques, such as those presented below.
An assembling method is known, in which surface scratches are prevented by applying a cylindrical protective sleeve on the fuel rod over its entire external surface before its insertion into the grid cells, and removing the sleeve after the insertion is completed. (Japanese Patent Application, First Publication, S53(1978)-11294)
Another approach is to insert a slitted cylindrical jig in the grid cell beforehand, and after placing the fuel rod in the jig, the jig is moved into the assembly with the fuel rod, and the jig is removed with the use of the slit. (Japanese Patent Application, First Publication, S60(1985)-50490)
Another method is to arrange a plurality of such slit tools in a line along the grid cells, and to insert a line of fuels rods all at once. (Japanese Patent Application, First Publication, S60(1985)-67891)
Yet another method is to insert a cylindrical plastic sheet into the grid cell prior to inserted a fuel rod in the plastic sheet. (Japanese Patent Application, First Publication, S60(1985)-181680)
Yet another method is to prepare a rattan type support made of a series of thin plastic strips, and arrange the supports in accord with the grid cells prior to inserting the fuel rods into the support. (Japanese Patent Application, First Publication, S60(1985)-213889)
Such method which utilize various protective covers between the grid cells and the fuel rods are effective in preventing the formation of surface scratches on the fuel rods, but none of them provided for an automated facility for insertion of a plurality of fuel rods efficiently. Therefore, the operation was labor-intensive, and a need existed to reduce the labor content associated with such operations.